This invention relates to a radar device which transmits a signal which is varied in frequency and modulated in order to avoid detection, avoid the possibility of "smart" jamming if detected and in order to reduce power requirements of the output signal. Typically, such radar devices are used as quiet altimeters for unmanned aircraft having a terrain following capability, as well as other terrain following aircraft. Quiet altimeters are also useful for other applications, including altitude sensors and non-military aircraft.
A radar altimeter, in a simple form, transmits pulses of radar energy, with ranging information being obtained in the classical manner by measuring a delay time of an echo signal. In order to provide a continuous wave transmission, the transmitted signal must be modulated and the delay in modulation be somehow measured. For example, an FM radar frequency modulates the transmitted signal and measures the frequency shift of the echo in order to obtain range information. The continuous transmission also creates considerable noise, including noise resulting from transmitter leakage signals during echo reception. In order to overcome these noises, a substantial output must be employed and appropriate filters.
In military operations, the radar signal not only provides an enemy with information concerning the existence of a source of a transmitted signal but also may enable the enemy to provide false information in the form of imitation echo signals. For this reason, various methods are used to reduce the power output of radar altimeters and to introduce random or pseudo-random patterns to the radar-transmitted signal. One of the problems with random and pseudo-random signals is that the echo signals no longer fall within a very narrow frequency range, making it difficult for a receiver to be designed only for receiving signals at a desired frequency. In order to eliminate spurious signals, elaborate filtering techniques are required. The transmitted signal must therefore have sufficient amplitude to overcome problems caused by spurious signals being received. In a continuous wave radar transmission, a leakage signal occurs, usually between the transmitting and receiving antenna. This leakage signal must be filtered out or otherwise ignored in order to process the desired echo signals. In randomly modulated radar, the means to filter or ignore leakage signals becomes increasingly complex. Again, the receiver requirements imposed by a need to filter leakage signals tends to increase the transmitted power necessary to insure proper reception.
In unmanned aircraft applications, as well as in most other applications, various terrain is likely to be encountered. This results in varying requirements for power output in order that the altimeter's radar can "read" the terrain. Variations in the power requirements also exist as a result of differences in mean altitude. This means that a radar altimeter need not transmit a predetermined maximum amount of power during all times of its operation.
It is accordingly an object of the present invention to provide a radar altimeter which is capable of producing a randomly modulated output and which operates at a minimum power level. It is a further object to provide for a means for interpreting the echos of such an altimeter by referring to the modulation of the transmitted signal. It is a further object to include a means for eliminating as many spurious signals from the received echos as possible by utilizing the above-mentioned means for interpreting the echos as a part of a filter circuit. It is a further object to provide a randomly modulated radar altimeter which simply and accurately selects a proper return signal in order that a transmitted signal may be provided at a low power output. It is a further object to provide an altimeter which is able to adjust its power output in accordance with the minimum power required for adequate reception of radar signals during the particular circumstance of operation.